Integrated circuit fabrication method

ABSTRACT

A method for forming an anti-reflective coating useful in the fabrication of integrated circuits is discussed. Applicants have found that preheating semiconductor wafers prior to the application of amorphous silicon anti-reflective coatings tends to reduce undesirable particulates which may attach to the wafer. The process is illustratively performed in a Varian 3180 machine having four stations. Illustratively, the wafer may be preheated between 70° C. and 175° C. prior to and during the sputter deposition of an amorphous silicon anti-reflective coating.

TECHNICAL FIELD

This invention relates to methods for fabricating semiconductorintegrated circuits.

BACKGROUND OF THE INVENTION

Many processes utilized for the fabrication of integrated circuitsinvolve the formation, by sputtering or deposition, of a material layer.A photoresist is spun on top of the material layer and subsequentlypatterned. The patterned photoresist serves as a mask through which thematerial layer may be etched using either dry or wet processes.

Anti-reflective coatings (ARC) are often positioned between the materiallayer and the photoresist. Anti-reflective coatings absorb the lightwhich is used to expose the photoresist and prevent undesiredreflections which might inadvertently expose portions of the photoresistwhich should not be exposed. Amorphous silicon is frequently employed asan anti-reflective coating over materials such as aluminum, spin-onglass, oxides, etc.

SUMMARY OF THE INVENTION

Amorphous silicon anti-reflective coatings are frequently deposited bysputtering utilizing sputtering apparatus such as a Varian 3180 or othermachine. Applicants have frequently noticed that after formation of ananti-reflective coating, many small particles having sizes betweenone-half and two microns may be observed on the wafer. The compositionof the particles has not been determined by applicants. It ishypothesized that the particles may be silicon or another material.These excess particles are undesirable because they can cause shortsbetween runners or may ultimately affect integrated circuit reliability.

The problem of undesirable particles may be alleviated by the presentinvention which illustratively includes preheating the substrate priorto formation of the anti-reflective coating. Desirably, the substrate ismaintained at the same preheat temperature during the process of formingthe coating. As explained below, the preheating creates mechanisms whichtend to discourage particles from sticking to the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a portion of a partially fabricatedintegrated circuit useful in understanding an illustrative embodiment ofthe present invention; and

FIG. 2 is a schematic diagram illustrating an apparatus or machineemployed in an illustrative embodiment of the present invention.

DETAILED DESCRIPTION

In FIG. 1, reference numeral 11 denotes a substrate which mayillustratively be aluminum, spin-on glass, or a form of silicon dioxide(for instance, either a thermal oxide or a deposited oxide). In general,the term substrate refers to any material upon which other materiallayers may be formed.

Reference numeral 13 denotes an anti-reflective coating which is,illustratively, amorphous silicon. Layer 13 has been in the past,typically, formed by sputtering at room temperature in, for example, aVarian 3180 machine.

Applicants have discovered that preheating substrate 11 prior to andduring deposition of anti-reflective coating 13 tends to reduce thenumber of particles upon layer 13.

FIG. 2 is a diagram which schematically represents a Varian 3180 sputterdeposition machine 50 commercially available from Varian Associates.Reference numeral 31 denotes a load lock through which wafers enter andleave the machine. Reference numerals 15, 17, 19, and 21 denote each ofthe four stations typical of the Varian 3180 machine. Reference numerals23, 25, 27, and 29 represent individual wafers positioned in each ofstations 15, 17, 19, and 21, respectively. In one illustrativeembodiment of applicants' invention, stations 15, 17, 19, and 21 areeach maintained at 175° C.±2° C. Each station 15, 17, 19, and 21 iscapable of heating an individual wafer to a predetermined temperaturebecause each station has a heater similar to a hot plate to which thewafer is clamped by wafer clips. Sputter deposition of ananti-reflective coating of amorphous silicon (similar to referencenumeral 13 of FIG. 1) is performed at station 21 upon wafer 29. However,wafer 29 has been preheated to 175° C. by its passage through each ofstations 15, 17, and 19, respectively. Of course, as is typicalproduction practice, after sputtering of an anti-reflective coating iscompleted upon wafer 29, wafer 29 is removed via loadlock 31, and wafer27 proceeds to station 21 where deposition commences upon wafer 27.Wafer 27 has, of course, been preheated to approximately 175° C.

As mentioned before, sputter deposition of an amorphous siliconanti-reflective coating is ordinarily performed at room temperature.(Deposition of other silicon layers, for example, gate polysilicon, maybe performed upon the preheated wafers because there is concern aboutthe mechanics of grain growth. But, typically, those who formanti-reflective coatings have no interest in grain size and consequentlysputter at room temperature.)

Applicants hypothesize that preheating of wafers such as wafer 29 beforesputter deposition prevents the deposition of particles uponanti-reflective coating 13 because the preheating in the Varian machinemay establish convection currents around the wafer which tend to carryparticles away from it. Furthermore, the comparatively cold particlespresent in the ambient environment of machine 50 tend to attach tocolder surfaces, rather than to the preheated wafer surfaces.Furthermore, applicants believe that a thermophoresis phenomenon is alsopartially responsible for preventing particles from clinging to thewafer surfaces. Applicants believe that the particles may be heatedduring the sputter deposition process in such a way that only one sideof the particle has a higher temperature than the other. An instabilityresults in which the particles oscillate and tend to be sucked into thesystem vacuum pump rather than settling upon the heated wafer.

In the embodiment discussed above, wafer 29 is originally introduced viaload lock 31 to heated station 15. Wafer 29 already has a layer ofaluminum or spin-on glass or an oxide formed upon it.

In another embodiment of the present invention, aluminum may bedeposited upon wafers in machine 50 at a station such as station 17, andan anti-reflective coating may be deposited upon the wafers at a stationsuch as station 21. In this embodiment, a wafer is introduced via loadlock 31. The wafer may or may not be preheated at station 15. When thewafer is introduced to station 17, aluminum deposition at temperaturesnear 300° C. is performed. After deposition, the wafer moves to station19 where it is heated to a temperature of approximately 70° C.±2° C.After the wafer reaches thermal equilibrium, it is transferred tostation 21 where an anti-reflective coating is deposited by sputterdeposition at a temperature of 70° C.±2° C. A temperature of 70° C.(lower than the preheat temperature of 175° C. discussed in theembodiment above) is utilized to avoid the possibility of the amorphoussilicon anti-reflective coating 13 spiking into the underlying aluminumsubstrate. The lower preheat temperature is believed to retard suchspiking.

In general, the temperature range between 175° C. and 70° C. has beenfound to produce satisfactory results.

We claim:
 1. A method of semiconductor integrated circuit fabricationcomprising:sputtering an amorphous silicon anti-reflective coating upona substrate chosen from the group consisting of aluminum, spin-on glassand silicon dioxide, by preheating said substrate to a temperaturebetween 70° C. and 175° C. prior to said sputtering step and maintainingsaid temperature during said sputtering.